As a conventional liquid vaporizing apparatus, description will be made about a chemical gas-phase growth apparatus that vaporizes liquid material to use it as reaction gas so as to form a silicon oxide film, and, more particularly, about a liquid vaporizing apparatus for use in a process which uses a reaction gas consisting of gas formed by vaporizing tetraethylorthosilicate (hereinafter called "TEOS"), a gas for adding P (phosphorus) into the film and formed by vaporizing trimethyl orthophosphate (hereinafter called "TMPO"), a gas for adding B (boron) into the film and formed by vaporizing triethyl borate (hereinafter called "TEB"), nitrogen gas and partially-ozonized oxygen gas, the liquid vaporizing apparatus vaporizing liquid by flowing the gas into the liquid heated and maintained higher than the room temperature.
As the foregoing chemical gas-phase growth apparatus, a chemical gas-phase growth apparatus for forming a silicon oxide film will be described. As a vaporizer, a vaporizer will be described in which the gas flows into liquid heated and maintained to a temperature higher than the room temperature, so as to vaporize the liquid by bubbling.
First, film forming reactions to be realized in the conventional chemical gas-phase growth apparatus will now be described. A chemical gas-phase growth apparatus of the foregoing type is constituted, for example, as shown in FIG. 50. The foregoing chemical gas-phase growth apparatus will now be described with reference to the foregoing figure. Referring to FIG. 50, the conventional chemical gas-phase growth apparatus comprises: a reaction chamber 1 for forming a film on a semiconductor wafer (omitted from illustration); an oxygen-gas supply pipe 2; an oxygen-gas flow-rate adjuster 3 for measuring and adjusting the flow rate of the oxygen gas flowing through the oxygen-gas supply pipe 2; an ozone generator 4 for ozonizing a portion of the oxygen gas; an ozone-gas introduction pipe 5 for supplying, to the reaction chamber 1, the partially-ozonized oxygen gas from the ozone generator 4; a liquid vaporizing apparatus 6 to be described later; a reaction-gas introduction pipe 7 for supplying, to the reaction chamber 1, the reaction gas from the liquid vaporizing apparatus 6; a heater 8 for heating and maintaining the hot state of the reaction-gas introduction pipe 7; and an exhaust pipe 9 for discharging the gas from the reaction chamber 1.
In the thus-constituted conventional chemical gas-phase growth apparatus, the ozone gas from the ozone-gas introduction pipe 5 and the reaction gas from the reaction-gas introduction pipe 7 are supplied onto a semiconductor wafer (omitted from illustration) heated and maintained by a wafer stage (omitted from illustration) so that they are made to react with each other. Thus, a film (omitted from illustration) is, by the reaction, formed on the semiconductor wafer, while the gas is, after the reaction, discharged outside from the reaction chamber 1 through the reaction pipe 9.
Then, a liquid vaporizing apparatus 6 of the foregoing conventional chemical gas-phase growth apparatus shown in FIG. 50 will now be described. A liquid vaporizing apparatus of the foregoing type is constituted, for example, as shown in FIG. 51 which is a view showing the system of piping of the same. Then, the liquid vaporizing apparatus will now be described with reference to the foregoing figure. Referring to FIG. 51, the liquid vaporizing apparatus 6 comprises: a plurality of vaporizers 10a, 10b and 10c for vaporizing TEOS liquid, TMPO liquid and TEB liquid; nitrogen-gas flow rate adjusters 11a, 11b and 11c for measuring and adjusting the flow rate of the nitrogen gas to be supplied to the vaporizers 10a, 10b and 10c; a nitrogen-gas flow rate adjuster 11d for measuring and adjusting the flow rate of the nitrogen gas serving as a carrier gas; a nitrogen gas supply pipe 12 for supplying the nitrogen gas to the liquid vaporizing apparatus; liquid-supply pipes 13a, 13b and 13c for supplying, from liquid-material supply containers (omitted from illustration) for storing the TEOS liquid, the TMPO liquid and the TEB liquid liquids, the liquids to the corresponding vaporizers 10a, 10b and 10c; gas introduction pipes 14a, 14b and 14c for introducing, into the vaporizers 10a, 10b and 10c, the nitrogen gas, the flow rate of which has been adjusted by the nitrogen-gas flow rate adjusters 11a, 11b and 11c; gas ejection pipes 15a, 15b and 15c for ejecting the nitrogen gas and the vaporized gas from the gas introduction pipes 14a, 14b and 14c and the vaporizers 10a, 10b and 10c to supply the gases to the reaction-gas introduction pipe 7; bypass pipes 16a, 16b and 16c for establishing the connections between the corresponding gas introduction pipes 14a, 14b and 14c and the gas ejection pipes 15a, 15b and 15c; drain pipes 17a, 17b and 17c for discharging the liquid in the vaporizers to a waste-liquid pipe (omitted from illustration); valves 18a to 22c provided in each pipe; the reaction-gas introduction pipe 7 communicated with the nitrogen-gas flow rate adjuster 11d and joined to each of the gas ejection pipes 15a, 15b and 15c to supply the reaction gas from the liquid vaporizing apparatus 6 to the reaction chamber 1 shown in FIG. 50; and a heater 8 for heating and maintaining the hot state of the gas introduction pipes 14a, 14b, 14c, the gas ejection pips 15a, 15b and 15c, the bypass pipes 16a, 16b and 16c and the reaction-gas introduction pipe 7 in cooperation with a temperature sensor (omitted from illustration) for detecting their temperatures.
In the chemical liquid vaporizing apparatus shown in FIG. 51, the valves 21a, 21b and 21c are opened to introduce predetermined liquid materials (omitted from illustration), through the liquid-supply pipes 13a, 13b and 13c, to the vaporizers 10a, 10b and 10c heated and maintained at a predetermined temperature level. Then, the valves 18a, 18b and 18c and the valves 20a, 20b and 20c are opened, while the valves 19a, 19b and 19c are closed. Thus, the nitrogen gas of a predetermined quantity controlled by the nitrogen-gas flow rate adjusters 11a, 11b and 11c is introduced from the gas introduction pipes 14a, 14b and 14c into the vaporizers 10a, 10b and 10c so that the foregoing liquid materials are vaporized. The vaporized gas formed due to vaporization of the liquid materials performed in the vaporizers 10a, 10b and 10c and the nitrogen gas introduced into the same are ejected through the gas ejection pipes 15a, 15b and 15c. Then the nitrogen gas flow rate adjuster 11d controls the flow quantity of the gas to a predetermined quantity, the gas being then joined to the nitrogen gas flowing through the reaction-gas introduction pipe 7 to be supplied to the reaction chamber 1 shown in FIG. 50.
In order to prevent change in the temperature of the liquid occurring at this time due to blowing of the low-temperature nitrogen gas into the liquid materials heated and maintained in the vaporizers 10a, 10b and 10c, the gas introduction pipes 14a, 14b and 14c are always heated and maintained by the heater so that the nitrogen gas to be introduced has the same temperature as that of the foregoing liquid materials. The gas ejection pipes 15a, 15b and 15c and the reaction gas introduction pipe 7 are, by the heater 8, always heated and maintained at a temperature higher than the temperature at the vaporization in order to prevent re-liquefaction of the TEOS vaporized gas occurring due to lowering of the temperature of the TEOS vaporized gas in the foregoing pipes.
If the vaporization of the liquid materials is not performed, the valves 19a, 19b and 19c are opened and the valves 18a, 18b and 18c and the valves 20a, 20b and 20c are closed so that the nitrogen gas is not flowed to the vaporizers 10a, 10b and 10c but the same is flowed to the bypass pipes 16a, 16b and 16c to be introduced into the gas ejection pipes 15a, 15b and 15c so that only the nitrogen gas is introduced into the reaction gas pipe 7.
As a vaporizer of the conventional liquid vaporizing apparatus shown in FIG. 51, the vaporizer 10a using the TEOS liquid as the liquid material will now be described, the vaporizers 10b and 10c using the TMPO liquid and the TEB liquid being omitted from description because their elements and operations are the same as those of the vaporizer 10a. A vaporizer of the foregoing type is constituted, for example, as shown in FIG. 52 which is a cross sectional view. Then, a description will be made about the foregoing vaporizer with reference to the foregoing figure. Referring to FIG. 52, in a container 23 for accommodating TEOS liquid A, there are disposed a temperature sensor 24 for detecting the temperature of the TEOS liquid A and a liquid-surface sensor 25 for detecting liquid surface B of the TEOS liquid A. The liquid-surface sensor 25 has, at the leading portion thereof, a conical liquid-surface detection portion 26 for detecting a state of contact with the liquid surface B. Reference numeral 27 represents a heater for heating and maintaining the TEOS liquid A at a predetermined temperature level by using the temperature sensor 24. Also shown in FIG. 51, on the top surface of the container 23, there are connected a liquid-supply pipe 13a having a valve 21a, a gas introduction pipe 14a having a valve 18a, a gas ejection pipe 15a having a valve 20a and a pressure gauge 30, the foregoing pipes being communicated with the inside of the container 23. At the leading portion (the lower end) of the gas introduction pipe 14a, a bubbling pipe 28 for blowing nitrogen gas into the TEOS liquid A is integrally connected, the bubbling pipe 28 penetrating the top surface of the container 23 to extend to the inside of the container 23 while having a mesh 29 disposed at the leading portion thereof. A drain pipe 17a having a valve 22a is connected to the lower portion of the container 23. In the container 23, internal space C is formed above the liquid surface B so that the pressure of the internal space C is detected by the pressure gauge 30. The gas ejection pipe 15a has a pressure adjuster 31 for adjusting the pressure of the internal space C in the container 23 in accordance with the pressure level detected by the pressure gauge 30.
The TEOS liquid A is, prior to the vaporization, injected into the vaporizer 10a shown in FIG. 52 by opening the valve 20a and 21a and by closing the valves 18a and 22a until the liquid surface B in the container 23 reaches the liquid-surface detection portion 26 of the liquid-surface sensor 25. Then, the valves 20a and 21a are closed. The temperature of the TEOS liquid A in the container 23 is detected by the temperature sensor 24 so that it is heated and maintained at a predetermined temperature level by the heater 27.
When the vaporization is performed, the valves 18a and 20a are opened in a state where valves 21a and 22a are closed so as to blow the nitrogen gas into the TEOS liquid A in the container 23 through the mesh 29 disposed at the leading portion of the bubbling pipe 28. The blown nitrogen gas is formed into bubbles D in the TEOS liquid A so that the TEOS liquid A is vaporized in the bubbles D during rise of the bubbles D in the TEOS liquid A to the liquid surface B. The vaporization is continued until the vaporized TEOS gas is saturated in the nitrogen gas in the bubbles D. When it is saturated, further vaporization is interrupted. The quantity of the vaporized TEOS gas can be expressed as follows: EQU G.sub.T =P.sub.T +(P-P.sub.T).times.G (1)
In Equation (1), GT is the quantity of the vaporized TEOS gas, G is the quantity of the nitrogen gas blown in the TEOS liquid A, PT is the vapor pressure of the TEOS liquid A heated and maintained at a predetermined temperature level and P is the pressure of the nitrogen gas in the bubbles D, the pressure being the same as the pressure of the internal space C in the container 23.
When the bubbles D reach the liquid surface B, the nitrogen gas in the bubbles D and the vaporized TEOS gas are ejected into the internal space C in the container 23 and then flow from the internal space C in the container 23 through the gas introduction pipe 15 so as to be ejected to the outside of the container 23.
When the vaporization is not performed, the valve 18a is closed to inhibit blowing of the nitrogen gas into the TEOS liquid A in the container 23. Since also the foregoing vaporization takes place from the liquid surface B to the internal space C in the container 23 until the partial pressure is made to be the same as the vapor pressure of the foregoing TEOS liquid A, the valve 20a is closed to inhibit the ejection of the vaporized TEOS gas from the container 23.
When the TEOS liquid A is discharged from the container 23, the valves 18a and 22a are opened in a state where the valves 20a and 21a are closed to blow the nitrogen gas into the container 23 through the bubbling pipe 28 so as to, with pressure, send and discharge the TEOS liquid A through the drain pipe 17a.
Although the vaporizer shown in FIG. 52 has been described as an example of the conventional vaporizer, there is a vaporizer 10a' shown in FIG. 53 serving as another example. Then, the foregoing vaporizer will now be described with reference to the foregoing figure. The description of the same structures and operations as those of the vaporizer shown in FIG. 52 are omitted here.
Referring to FIG. 53, reference numeral 32 represents a thermostatic chamber accommodating the container 23, the liquid-supply pipe 13a, the gas introduction pipe 14a, the gas ejection pipe 15a, the drain pipe 17a, their valves 18a, 20a, 21a and 22a, the pressure gauge 30 and the pressure adjuster 31. The thermostatic chamber 32 has, in the upper portion thereof, a temperature sensor 33 for detecting the temperature of the inside portion, while the thermostatic chamber 32 has, on the inner surface of the side wall thereof, a heater 34 for heating and maintaining the inside portion of the thermostatic chamber 32 at a predetermined temperature level in accordance with the temperature detected by the temperature sensor 33. In the vaporizer 10a' shown in FIG. 53, the elements in the thermostatic chamber 32 such as the container 23 and the TEOS liquid A in it are heated and maintained at a predetermined temperature level by the heater 34. Therefore, the temperature sensor 24 and the heater 27 shown in FIG. 23 are not provided.
Also in the vaporizer 10a' shown in FIG. 53, the injection of the TEOS liquid A into the container 23 and the vaporization of the TEOS liquid A in the container 23 and the discharge of the TEOS livid A from the container 23 are performed identically to the foregoing vaporizer 10a shown in FIG. 52.
In each of the vaporizer 10a shown in FIG. 52 and the vaporizer 10a' shown in FIG. 53, the liquid surface B is lowered as the vaporization of the TEOS liquid A proceeds in the container 23. Therefore, the vaporization is interrupted at a certain moment and TEOS liquid A is again supplied to the liquid-surface detection portion 26 of the liquid-surface sensor 25 in the container 23 by a method identical to the foregoing method.
In the foregoing conventional vaporizer 10a shown in FIG. 52, the heater 27 provided for the container 23 is disposed on the side surface of the lower portion of the container 23. It leads to a fact that a vertical temperature difference of the TEOS liquid A occurs when no vaporization is performed. In this way the temperature of the upper portion is made to be higher than that of the lower portion. If the vaporization is performed in the foregoing state, the TEOS liquid A is stirred by the bubbles D, resulting in that the upper and lower portions having different temperatures are mixed. As a result, the temperature of the overall body of the TEOS liquid A is changed. An example of the results of a measurement of the temperature change is shown in FIG. 54.
If temperature change of the TEOS liquid A of the foregoing type takes place, the vapor pressure of the TEOS liquid A shown in Equation (1) is changed. Hence, the quantity of the vaporized gas is changed. If the quantity of the vaporized gas is changed, the vaporized gas changes the speed of the growth of the film to be formed on the semiconductor wafer due to the reactions. If the film formation is inhibited during the change in the temperature of the TEOS liquid A, the temperature of the TEOS liquid A in the container 23 is restored to an initially-set temperature and, therefore, the film formation must wait. As a result, the time taken to form the film is lengthened.
In the vaporizer 10a shown in FIG. 52, lowering of the liquid surface B occurring as the TEOS liquid A in the container 23 is vaporized reduces the quantity of the vaporized gas of the TEOS liquid A. FIG. 55 shows the change of the quantity of the vaporized TEOS gas as the time passes from the commencement of the vaporization due to bubbling. As can be understood from the figure, the vaporization, which takes place due to bubbling, lowers the liquid surface B and reduces the quantity of the vaporized TEOS gas.
Therefore, the reduction in the quantity of the vaporized TEOS gas lowers the growth speed of the film to be formed on the semiconductor wafer by the vaporized gas. FIG. 56 is a graph showing the relation between the film growth speed and the quantity of the TEOS liquid in the container 23 serving as the vaporizer. As can be understood from the figure, the film growth speed is lowered, dependent on the quantity for the liquid.
One of main reasons of the reduction in the quantity of the vaporized gas taking place as the liquid surface B lowers is that: there is escape of heat to the outside portion of the container 23 from the internal space C in the container 23 through the temperature sensor 24, the liquid-surface sensor 25 and the bubbling tube 28, the escape of the heat being augmented because the exposure to the internal space C in the container 23 of the temperature sensor 25 and the bubbling tube 28 increases as the liquid surface B lowers so that the temperature of the internal space C in the container 23 is made to be lower than the temperature of the TEOS liquid A so that the TEOS liquid A in the internal space C in the container 23 is again liquefied, causing the quantity of the vaporized gas to be reduced.
Another main reason of the reduction in the quantity of the vaporized gas as the liquid surface B lowers is that the internal space C in the container 23 is enlarged as the liquid surface B is lowered, the area of the gas in the internal space C in the container 23 which is in contact with the side wall of the container 23 is enlarged and the escape of the heat from the internal space C in the container 23 to the outside portion of the container 23 is augmented, resulting in that the temperature of the internal space C in the container 23 is made to be lower than the temperature of the TEOS liquid A and that the TEOS liquid A in the internal space C in the container 23 is again liquefied, causing the quantity of the vaporized gas to be reduced.
As contrasted with the fact that the foregoing problems are experienced with the vaporizer shown in FIG. 52, the vaporizer shown in FIG. 53 is free from the augmentation of the heat escape from the internal space C in the container 23 to the outside portion of the container 23 even if the liquid surface B is lowered. Therefore, the reduction in the quantity of the vaporized gas due to the re-liquefaction of the TEOS liquid A does not take place. However, the inside portion of the actual thermostatic chamber 32 is not fully and always heated and maintained at a predetermined temperature level. In particular, the temperature of the portions apart from the temperature sensor 33 and the heater 34 is lower than the predetermined temperature, resulting in heat escape from the internal space C in the container 23 to the outside portion of the container 23. As a result, TEOS liquid A in the internal space C in the container 23 is again liquefied, causing the quantity of the vaporized gas to be reduced.
Under a circumstance that the TEOS liquid A must be, at predetermined intervals, supplied into the container 23 of each of the foregoing conventional vaporizers shown in FIGS. 52 and 53, if the temperature of the TEOS liquid A to be supplied is not the same as the temperature of the TEOS liquid A heated and maintained at a predetermined temperature level in the container 23, the temperature of the TEOS liquid A in the container 23 is changed after the liquid supply. The change in the liquid temperature, as described above, changes the quantity of the vaporized gas which changes the growth speed of the film to be formed due to the reactions and which requires a waiting time for the formation of the film to be taken.
As for the temperature change, the vaporizer shown in FIG. 53 and having the arrangement that the container 23 and the TEOS liquid A are heated and maintained at a predetermined temperature level in the thermostatic chamber 32 does not easily encounter the vertical temperature difference of the TEOS liquid A as is experienced with the vaporizer shown in FIG. 52 because the overall body of the container 23 is heated by the ambient atmosphere. Since heating of the container 23 is not performed by a heater attached directly, the time taken to restore the original controlled-temperature is lengthened as compared with the time required for the vaporizer shown in FIG. 52.
In order to prevent the foregoing liquid change, a vaporizer serving as another conventional example and shown in FIG. 57 comprises a reservoir tank 35 disposed in the thermostatic chamber 32 so as to supply, to the container 23, the TEOS liquid A heated and maintained, in the reservoir tank 35, at the same temperature as the TEOS liquid A in the container 23.
However, the foregoing method involves a fact that the size of the thermostatic chamber 32 cannot be reduced. Moreover, if liquid, the temperature of which is lower than the internal temperature of the thermostatic chamber 32, is injected into the reservoir tank 35, the temperature in the thermostatic chamber 32 is undesirably lowered, resulting in that the quantity of the vaporized gas is reduced. Therefore, a long time is required to restore the original controlled-time.
In a case where the mesh 29 is not used in the bubbling tube 28, vaporization takes place in a state where each bubble D has a large volume and the number of the bubbles D is small as shown in FIG. 58. Since the area of the nitrogen gas, which is in contact with the TEOS liquid A, per volume is small at this time, the TEOS is not vaporized sufficiently to saturation.
If the vaporization is performed sufficiently to the saturation level, the quantity of the saturated vapor cannot be changed although the time during which the nitrogen gas in the bubbles D is in contact with the TEOS liquid A is shortened. In a state where the saturation is not achieved or in a state where the saturation is at the very limit, the liquid surface B is lowered, and the time taken for the bubbles D to rise in the TEOS liquid A is reduced. Therefore, the contact time of the nitrogen gas in the bubbles D and with the TEOS liquid A is shortened. Therefore, the vaporization state does not reach saturation and therefore the quantity of vapor sent from the vaporizer is reduced.
If the distance which the bubbles D rise in the TEOS liquid A is lengthened so as to sufficiently lengthen the time in which the nitrogen gas in the bubbles D comes in contact with the TEOS liquid A, saturation is achieved. Therefore, the reduction in the quantity of vapor taking place due to lowering of the liquid surface can be prevented. However, the foregoing method involves a necessity of enlarging the height of the container, causing a problem in that the size of the vaporizer cannot be reduced.
If the vaporization has not sufficiently reached saturation, vaporization, as shown in FIG. 59, takes place from the liquid surface B, liquid droplets E generated when the bubbles D have burst at the liquid surface B and are to adhered to the internal surface of the container 23 in the internal space C in the container 23 and mist F is generated similarly and present in the vaporized gas. Since the states of the liquid surface B, the liquid droplets E and the mist F are not stable in constant conditions, the quantity of the vapor from the vaporizer can change.
If the vaporization has not sufficiently reached the saturation level, the usual quantity of the vaporized gas is smaller than the quantity realized in a saturated state. In a period in which the vaporization is not performed by bubbling, the TEOS liquid A is vaporized from the liquid surface B into the internal space C in the container 23, causing the vaporized TEOS gas, which has been saturated, to be accumulated in the internal space C in the container 23. Therefore, the saturated and vaporized gas, which has been accumulated, flows out and the state shown in FIG. 55, in which the quantity of the vapor is very large, is continued for the while until the vapor quantity is made to be a usual non-saturated quantity.
If the quantity of the nitrogen gas to be blown into the TEOS liquid A is enlarged or if the enlargement of the bubbles D considerably changes the liquid surface B, the quantity of the generated liquid droplets E and the mist F are enlarged proportionally. If the quantity of the generation is enlarged, a portion of it is ejected from the container 23 to the gas ejection pipe 15a and the liquid-supply pipe 13a, resulting in that the ejected portion is allowed to adhere to and accumulated on the inner surface of the piping. The accumulated portion is again vaporized and flowed into the internal space C in the container 23, causing the quantity of the vapor to be changed.
Accordingly, the conventional vaporizer comprises, as shown in FIG. 60 in an enlarged manner, the mesh 29 disposed at the leading portion of the bubbling tube 28 to make small the bubbles D of the nitrogen gas to be blown into the TEOS liquid A in order to bring the vaporization closer to saturation so as to prevent change in the liquid surface B at the bursting of the bubbles D so that generation of the liquid droplets E and the mist F is prevented or minimized. However, the actual vaporizer comprising the mesh 29 having fine squares results in that the bubbles join together on the surface of the mesh 29 and that large bubbles D are undesirably formed.
Since only one mesh 29 for blowing the nitrogen gas into the TEOS liquid A is provided, a very small portion of the TEOS liquid A is passed in a period in which the bubbles D reach the liquid surface B. The vaporization is performed in only a portion adjacent to the bubbling pipe 28, thus resulting in that the bubbles D join together in the TEOS liquid A and that the area of the nitrogen gas, which comes in contact with the TEOS liquid A, cannot be enlarged even if the flow rate of the nitrogen gas is enlarged. Therefore, saturation cannot easily be realized.
The foregoing conventional vaporizers 10a and 10a' shown in FIGS. 52 and 53 involve a necessity of stopping the vaporization at regular intervals to supply the TEOS liquid A to the container 23. The liquid supply is performed by the foregoing method so that the height of the liquid surface B is determined by the temperature sensor 25.
However, the detection of the liquid surface B performed by the temperature sensor cannot be performed sufficiently accurately such that the TEOS liquid A cannot be supplied to the same liquid surface with excellent reproducibility. Therefore, there arises the foregoing problem of the change in the quantity of the vapor occurring due to the change in the liquid surface.
Moreover, if the liquid-surface detection portion 26 of the liquid-surface sensor 25 cannot detect the liquid surface B because it is contaminated with the reaction products of the reactive TEOS liquid A, the TEOS liquid A flows over the container 23 through the gas ejection pipe 15a when the liquid is supplied as described above. The liquid flows from the vaporizer 10a, shown in FIG. 51, through the reaction-gas introduction pipe 7, causing a problem to rise in that the TEOS liquid A flows to the reaction chamber 1 shown in FIG. 50.
In order to overcome the foregoing problem, the foregoing conventional examples shown in FIGS. 52 and 53 comprise the liquid-surface-contact type sensor 25. In another method, for example, a sensor of a type for detecting the position of the liquid surface B by reflecting a laser beam G from a light emitter 37 from the liquid surface B and received by a light-receptor 38 involves a problem of the deterioration in the detection accuracy due to the shift of the liquid surface B and contamination of the light emitter 37 and the light receptor 38 interrupt the normal operation. A sensor of a type arranged as shown in FIG. 62 such that a float 38 is disposed on the liquid surface B to detect the position of the liquid surface B involves a similar deterioration in the detection accuracy due to the shift of the liquid surface B. Another risk is involved in that dust can be generated from a movable portions of the float 38. Therefore, the change of the type of the float does not enable accurate detection of the liquid surface.
Since the foregoing conventional vaporizers 10a and 10a' shown in FIGS. 52 and 53 comprise the vaporizing container 23 which is made of opaque material, the state of vaporization with nitrogen blown into the TEOS liquid A cannot be confirmed. Therefore, a variety of problems rise in that the changed state of the vaporization, such as the change in the size of the bubbles D and the state of the shift of the liquid surface B, cannot be confirmed, the changed stage being due to the adhesion of the reaction products of the TEOS liquid A to the mesh 29 or the like and breakage or the like due to degradation of the internal elements of the container 23.
The present invention is directed to overcome the foregoing problems and therefore an object is to provide a liquid vaporizing apparatus which is capable of accurately maintaining the liquid surface in a container for liquid to be vaporized by bubbling at a predetermined range.
Another object of the present invention is to provide a liquid vaporizing apparatus, arrangement of which is such that the vaporization by bubbling is made closer to saturation to prevent flying and adhesion of liquid droplets and change in the temperature of the liquid and the container are avoided to prevent a change in the quantity of vapor, so that reaction gas can further stably be supplied.
Another object of the present invention is to prevent change in generation of vaporized gas by maintaining the temperature of liquid in a container at a constant level without any change, by maintaining the temperature of a space above liquid in the container at a constant level without any change, and by maintaining the height of the liquid surface in the container at a constant value to prevent flying and adhesion of liquid droplets so as to maintain the vaporization state at a constant state.